Great Country Academician

Standing in the laboratory of Qixiashan Institute of Controllable Nuclear Fusion, Xu Chuan looked at the images and data on the monitor.

On the side of the laboratory, there is also an isolated experimental room.

Inside, scanning electron microscopes, metal in-situ analyzers, mass spectrometers and other equipment are analyzing the materials in the equipment.

The second limit experiment of the dawn fusion device created not only a two-hour high-density plasma operation record, but also a fusion ignition operation experiment of deuterium-tritium raw material.

The data and value brought by the real deuterium-tritium raw material fusion ignition operation experiment are not comparable to the high-density plasma flow operation simulated by helium three and hydrogen.

Although the latter can also be close to the former in terms of temperature and density, it cannot undergo fusion after all.

The former, even if it is only a milligram, can achieve real deuterium-tritium fusion to release energy, release neutrons, increase the temperature of the plasma, disrupt the operation of the plasma, and so on.

These are things that cannot be achieved by simulations of helium-3 and hydrogen.

Especially the neutron irradiation damage of the first wall material, which is the next world problem for controlled fusion relay to control the high temperature plasma turbulence in the reactor chamber.

The material of the first wall not only faces the high-temperature deuterium-tritium plasma in the reactor chamber, but also faces the neutron beam generated during the fusion process of deuterium-tritium raw materials.

In addition, the first wall material may even take on the function of tritium self-sustaining.

The two raw materials for DT controlled nuclear fusion are deuterium and tritium.

The content of deuterium element on the earth is huge. There are about 40 trillion tons of deuterium in seawater alone, and its production is relatively simple.

But compared with deuterium, the storage of tritium on the earth is quite scarce.

The stock of tritium in natural resources in the world is almost negligible, and the stock in nature is only about 3.5 kg.

At present, the storage of tritium raw materials in various countries does not exceed 25 kilograms in total.

On the one hand, tritium will emit beta rays autonomously and decay, and the half-life is only a short period of 12.5 years.

On the other hand, it can generally be prepared only through nuclear reactions.

At present, the industrial production of tritium mainly uses the neutrons of the reactor, uses lithium-6 compounds as targets to produce tritium, and then uses the thermal diffusion method to enrich the tritium to more than 99% before collecting and storing it.

The neutron beam is uncontrollable, and the amount produced in the nuclear fission reactor is not large, so the output is very low.

Therefore, in controllable nuclear fusion technology, how to keep tritium in a self-sustaining cycle is also one of the key issues.

Some people may think that a particle accelerator can be used to accelerate neutrons to bombard lithium materials to produce tritium materials, but those who have this idea are honestly basically those who did not study physics seriously in high school.

Neutrons carry no electrons, and the accelerator's magnetic field has no effect on them at all.

If the magnetic field could confine neutrons, the material for the first wall of a controlled nuclear fusion reactor would not be so difficult to find.

Fortunately, a large number of neutrons are produced during deuterium-tritium fusion. If neutrons are used to bombard the lithium-6 compound target, it is theoretically possible to maintain tritium self-sustaining.

During the operation of the Lixiao fusion reactor last time, Xu Chuan did such an experiment.

On the first wall, he installed lithium-6 compound targets, tungsten alloys, molybdenum alloys, graphite, carbon composite materials, beryllium alloys and other material sheets.

Among them, the lithium-6 compound target material is used to test whether the neutrons released during the deuterium-tritium fusion process can really bombard the lithium material as theoretically to produce enough tritium raw materials.

For other materials, it is to find the most suitable first wall material.

Neutron irradiation is no joke.

For now, it can have a strong transmutation effect on most materials and most metal materials.

This not only destroys the structure of the material, but also acts like a blowing agent, turning the material into a very fragile foam.

Imagine how it would feel to break a piece of steel as thick as a foam box into slag when you gently break it with your hands?

Neutron irradiation in a controlled fusion reactor can do just that.

In fact, this is also the case. Although the deuterium-tritium raw material used in the Dawn fusion device last time was only one milligram, the neutrons produced during the fusion process still have different degrees of impact on the various test materials deployed on the first wall. damage.

But it is gratifying that the lithium-6 compound target did play a corresponding role during the experiment, and some tritium elements were produced after the neutron beam produced by the fusion of deuterium and tritium hit it.

Therefore, it is theoretically possible to solve the problem of tritium self-sustainability by using lithium-6 compound targets as reactants.

This is also a major breakthrough.

After all, in the past, no experimental institution or research institution could truly use the experimental reactor to conduct deuterium-tritium fusion reaction to test neutron + lithium materials to synthesize tritium raw materials.

This should be their first time.

But there is good news, but there is more bad news.

The damage degree of the various test materials against neutron radiation installed on the first wall material is higher than what Xu Chuan calculated.

Looking at the images on the computer screen, Zhao Guanggui, a professor of materials science standing beside Xu Chuan, sighed softly, and said, "From the experimental data, there are many more problems than we imagined."

Xu Chuan looked at the images on the computer and said, "No matter how many things there are, we have to solve them one by one, right?"

Hearing this, Zhao Guanggui sighed: "That's true, but we have a lot of troubles. And we have now entered a new field. In the area of ​​controllable nuclear fusion, no other research institution or laboratory can do it." Provide us with experience as a reference."

Hearing this, Xu Chuan smiled and said: "Referring to other people's experience and ideas can indeed provide us with great convenience, but in the end it is just walking on the road of others. In terms of scientific research, if we want to achieve something , After all, you have to have your own ideas and ideas.”

"The way of being lazy may be suitable for other fields, but for us who are engaged in academic research, what to do and how to solve problems ultimately require our own independent thinking."

On the side, Xing Xuexing, a professor of materials transferred from Shuimu University, said with a smile: "Being able to go ahead and expand the boundaries is something that every researcher and scholar hopes for."

After a pause, he brought the topic back to the experimental data: "But what Professor Zhao said is correct, we have a lot of trouble this time."

"Whether it is tritium self-sustaining or the damage of various anti-neutron irradiation sample materials, it is far lower than expected before the experiment."

"It is indeed possible to generate tritium elements by bombarding lithium targets with neutrons. But the amount generated and the amount we collected are not as much as in theory."

"On the one hand, not all the neutron beams generated by fusion in the chamber act on the lithium-6 compound target. The energy it carries is too high, and it will directly break down the target, resulting in a much lower number of reactions than expected."

"On the other hand, the energy level carried by these neutrons is too high. At a temperature of 120 million degrees, the energy level of the neutron beam released by deuterium-tritium fusion is comparable to that of a medium-to-large particle collider. It has a very serious impact on both sides.”

Xu Chuan thought for a while, and said: "The first problem is easy to solve. At worst, the thickness of the target can be increased. In addition, it can be made into a full-coverage type, which wraps the reaction chamber as a whole, so that the neutron beam It's not wasted."

"As for the second one, it's a little troublesome."

Controlled nuclear fusion is not nuclear fission, and the temperature of nuclear fission is far lower than that of nuclear fusion.

Even if a large-yield nuclear bomb explodes, the temperature at the center will reach the sky-high level of millions of degrees Celsius.

When the little boy was dropped on Hiroshima, the temperature in the core area of ​​the explosion was only over 6,000 degrees. In contrast, this value is hardly worth mentioning in controlled nuclear fusion.

More than 6,000 degrees, this data is not even a fraction of the plasma temperature of the Dawn fusion device.

The temperature of nuclear bomb explosion is only like this, so the temperature of nuclear power plants that use nuclear fission effect to generate electricity is even lower.

Therefore, most of the anti-irradiation materials that can be used in nuclear fission reactors cannot be used in controllable nuclear fusion reactors at all.

Not only the lithium target used for tritium self-sustainment was damaged during the experiment, but other experimental materials deployed on the first wall were also damaged.

On the side, Zhao Guanggui tentatively said, "How about lowering the fusion temperature?"

"The temperature of deuterium-tritium fusion can occur at about 12 million degrees, and 120 million degrees, which is a full tenfold increase."

"Although lowering the temperature will affect the activity of the deuterium-tritium plasma, which in turn will affect the number of fusions and the energy generated. But it is not inadvisable to sacrifice part of the heat and energy in exchange for the stability of the first wall material."

Xu Chuan thought for a while, shook his head and said, "It's not very feasible."

"Although thermal motion can cause neutrons to collide inelastically, the higher the thermal motion speed, the greater the impact on matter, but the energy level of the neutron beam in the fusion reactor is not solely derived from temperature."

"Its main source is the energy boost produced by deuterium-tritium nuclear fusion. Each deuterium-tritium nuclear fusion will produce a neutron of 14.1 MeV. This part is destined in high-energy physics, and lowering the temperature is only a part of the external force. .”

Zhao Hongzhi nodded and said, "Well, from this point of view, it is basically impossible to reduce the temperature to reduce neutron damage to the first wall material."

"From the analysis data of materials after neutron irradiation, materials such as molybdenum, tungsten, and graphene are in the first step, and are less affected by neutron irradiation. Austenitic steel and ceramics are in the second step, and other worse."

On the side, Professor Xing Xuexing from Shuimu University shook his head and said: "Molybdenum is not good. The Mizuki side has done research before. When molybdenum is irradiated with neutrons, it will be transmuted into radioactive elements. As for molybdenum alloys, you need more. tried."

"But tungsten, tungsten alloy may still have some hope. At present, the first wall material of ITER and EAST uses tungsten alloy, which has good heat resistance. The transmutation products are osmium and rhenium, and there is no radioactive problem."

Xu Chuan shook his head and said, "Tungsten probably won't work either."

"There is nothing wrong with tungsten's heat resistance and transmutation products, but the difference in its physical plasticity and thermal expansion coefficient, as well as the accumulation of thermal stress, can cause cracks inside the material."

"This is fatal for a controlled fusion reactor."

Hearing that Xu Chuan rejected the tungsten alloy, the laboratory fell into silence again.

The problem of the material of the first wall is indeed very troublesome. It is so troublesome that no suitable one can be found in the whole world.

After all, in a controllable fusion reactor, the first wall material is strongly affected by high-energy neutrons, electromagnetic radiation and high-energy particles (deuterium, tritium, helium and other impurities) emitted from the plasma.

For a commercial tokamak reactor, in theory, the general neutron wall load should reach at least 5MW/m2.

The neutron wall load is a design index related to the power density of the fusion reactor, which is equal to the product of the fusion neutron source intensity and neutron energy per unit area of ​​the first wall material.

However, most heat-resistant materials simply cannot meet the requirements when faced with these extremely stringent property challenges.

But then again, if this problem is really so easy to solve, it won't stay until now.

After all, controllable nuclear fusion is something that the whole world has the ability to do. The various technical problems and material issues must have been discussed countless times.

Staring at the data on the computer screen, Xu Chuan pondered for a while and then said, "I think we need to change our thinking about the choice of materials for the first wall."

Hearing this, everyone else in the lab looked over.

Zhao Hongzhi asked: "How to say?"

Xu Chuan thought for a while, organized his words, and then said: "Every D-T fusion will produce a neutron of 14.1 MeV. Since the neutron is not charged, it cannot be confined by a magnetic field, and it will directly bombard the first wall material to cause damage. .”

"14.1 MeV is a very large energy. You must know that the atoms in the material are bound by various chemical bonds, and the bond energy is between 1 and 10 eV."

"That is to say, the energy carried by a 14.1 MeV neutron is enough to destroy millions of ordinary chemical bonds, which will undoubtedly cause irreparable damage to materials."

"In the fusion reactor, high-energy neutrons are like bullets fired at the material, constantly hitting the metal atoms, breaking the chemical bonds around them, forcing the atoms to leave their original positions, thus destroying the regular arrangement of atoms."

"If it is simply to resist neutrons, maybe structures made of materials such as beryllium gold, graphite, graphite and uranium 238 can do it. Isn't these materials used for neutron reflection in nuclear fission reactors?"

"But it won't work if you put it in a controllable nuclear fusion reactor."

"The reason is simple, because we need neutrons for tritium self-sustainment, otherwise the current stored tritium raw materials simply cannot support the commercial use of controlled nuclear fusion."

"So I personally think that instead of looking for a resistant material in metal materials, why not try to look at other materials?"